A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, it is necessary to measure parameters of the patterned substrate, for example the overlay error between successive layers formed in or on it. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. One form of specialized inspection apparatus is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometer are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle. Ellipsometers measure relative intensities of (portions of) radiation beams with different polarizations.
In scatterometers and lithographic apparatus, targets are used in the determination of overlay errors. These targets are conventionally positioned in the scribe lanes between the patterns. The overlay error at the target site is thus measured. However, the overlay error at the position of the pattern is therefore an interpolation from the measured overlay at different points surrounding the pattern, and not a measurement of the overlay on the pattern itself.
Although the targets could be positioned within the patterns themselves, this has the disadvantage that the targets used are relatively large and therefore take up too much of the are designed for product patterns, thereby compromising device functionality and taking up valuable “real estate”.
Similarly, targets are used in the determination of the profile of pattern structures. The accuracy of lithographic apparatuses is measured by measuring the size, shape (i.e. the profile), and dimensions (e.g. critical dimension (CD), side wall angle and other more complex dimensions from two-dimensional patterns) of structures within the target, properties of layers on the substrate such as layer thicknesses and material properties (n, k) of subsurface layers, and even geometrical properties from structures below a top level structure in the substrate, and this is extrapolated to the structures (and layers) of the pattern. As the behavior of the exposure portion of the lithographic apparatus can change over the surface of a substrate, this extrapolation is not always as accurate as would be desired.